Minimum Quantity Lubrication (MQL) is a process-sensitive approach to metal cutting lubrication. To achieve optimum results it is important to both know the amount of lubricant being applied and have the lubricant applied in a continuous or near-continuous manner. These two approaches have often been at odds with each other. Making the problem more difficult, is the wide range of flow rates required from the pump, such as 1 to 500 ml/hr., and the high pressure into which the pump must discharge fluid through the spindle system, up to 10 bar.
The standard industry approach to controlling the fluid flow is to use a pressurized fluid supply with a metering element that opens for a fixed period of time to allow a known amount of fluid to flow. To finely control this amount the metering element is opened and closed rapidly. To achieve the required accuracy, a manual calibration of the actual fluid flow through the specific metering element is required. Since fluid properties are dependent on fluid conditions, including viscosity, which changes with temperature, this approach has inherent difficulties. Attempts are being made at addressing these issues, for example, the development of “smart” valves (see DE102015005634A1), however the problems of consistency are inherent in the time based flow approach and such smart valves are much more complex than the inventive approach given in this disclosure.
An alternative to the time and flow approach is to use a volumetric pump. Because fluid viscosity changes with temperature, a metered volumetric dosing approach is often best for ensuring the amount of fluid delivered is consistent. Since it is volume based, it is insensitive to the fluid viscosity changes. There are several common designs of metering pumps, such as diaphragm, peristaltic, and gear pumps, but these generally cannot deliver the fluid at the full range of both rates and pressures necessary in the high-pressure, low flow, though-the-spindle metal-cutting environment. Some of these pumps can be practical in external MQL applications where the fluid is being discharged into atmospheric pressure, but the need to overcome the high compressed air back pressure inside a machine spindle make them impractical for through-the-spindle MQL systems. Others of these pumps can provide the necessary pressure, but only at much higher output rates than required for MQL applications.
Because of their ability to work reliably at the given conditions, several current MQL applicators use positive displacement single piston pumps. These pumps are typically operated with compressed air and deliver a low volume “shot” of fluid every time they stroke. They typically have outputs of about 0.03 mL/stroke and they are stroked repeatedly to generate the required flow rate. A negative aspect of this current implementation is that fluid is only delivered during the stroke phase of the pump and not as the pump chamber is refilled, so the output is pulsed. This is undesirable for consistent lubrication of the cutting tool and can cause issues when the cutting operation is shorter than the time between strokes.
To alleviate the pulsed output, operate at the required pressure, and deliver the small amounts of fluid needed, most current through-the-spindle MQL systems use a pressurized fluid supply with a metering element. This is typically done with a valve that opens and closes several times a second. Although this method provides an output which approximates continuous flow, it requires calibration for different liquids, is again sensitive to changes in viscosity and has a relatively large percentage of error at low flow rates.